These practices work. They change people. That is, they change mental experience, and those changes help people to experience God as more real. The practices don't work for everyone, and they do not work for each person to the same extent, but there are real skills involved here, skills that develop a psychological capacity called absorption that perhaps evolved for unrelated reasons, but that helps the Christian to experience that which is not materially present. These skills and practices make what is absent to the senses present in the mind.

To say this is not to say that God is an illusion. I am pointing out the obvious: that the supernatural has no natural body to see, hear, or smell. To know God, these Christians school their minds and senses so that they are able to experience the supernatural in ways that give them more confidence that what their sacred books say is really true.

It is a fragile process, because what they are doing is so hard, because it violates so much of what we take for granted. It takes an enormous amount of work. People must learn to see differently, and think differently, and above all feel differently, because for most people it will be a lifelong challenge to believe—to really feel as if they know in their heart of hearts—that God loves them as they are. When people build their understanding of God out of their own experience, they shape what they know of God's love out of the way they have experienced their mother's and father's love. But sometimes parents are not so loving, and always the love of a parent falls short of unconditional acceptance. The challenge of Christianity is being able to remap your own interior world from the way in which you learn to imagine God—and if it is hard to learn to experience yourself as truly in relationship with an invisible presence, it is harder still to experience yourself as feeling the love, tolerance, kindness, and forbearance you would feel if you truly, deeply, genuinely felt loved by the creator of the universe. Even when Christians succeed, they may grasp the moment—and then it may be gone.

Uncertainty remains at the heart of this process, as it has always done. Way back in the spring of 54 Anno Domini, Paul wrote from Ephesus to a church on the other side of the Aegean in the city of Corinth. He had founded the church some years previously in one of his evangelizing journeys around the Mediterranean, and it was now in trouble. Its members were squabbling about whether they could share meals with non-Christians, how they should settle disputes, whether marriage was appropriate for them—in short, about what it meant to be a Christian. Paul was a keenly pragmatic man who imagined believers as they could be but created a form for them as they were, ordinary folk who aspire, who stumble, and who often fall. But while he could solve the problem of whether people should marry and where they should take their conflicts, the church at Corinth had other troubles that Paul could not settle so easily.

In their passionate discovery of what they took to be the one true God, the Corinthians sought out moments when they thought that the supernatural divine broke into this mundane world. Those moments no doubt demonstrated to them that their god was real and lived among them still. Some of them spoke in tongues, giving voice in languagelike speech the speaker does not understand and believes to be divine. They seem to have paid excited attention to dreams, visions, and ecstatic transports. And they were arguing about how to interpret these experiences, how accurate they were, and what authority to give to those who experienced them. They were arguing, in short, about the most difficult problem that confronts anyone who believes or wants to believe in God: not whether God exists, in some abstract, in-principle, out-in-the-universe way, but how to find God in the everyday world and how to know that what you have found is God, and not someone else's deluded fantasy or your own selfish wish. For the Christians I met, the problem at the center of their faith is identifying the divine in ordinary life and distinguishing it from madness, evil, and simple human folly.